Mass spectrometers have played an important role in solar system exploration, having been employed to study the composition of atmospheres and solar system bodies on missions to the Moon, Mars, Venus, Jupiter, Saturn, Titan, and various comets. Mass spectrometers continue to be an important instrument for upcoming and planned NASA and European Space Agency missions.
An inherent challenge with conventional spaceflight mass spectrometers is the introduction of the material to be sampled (gas, solid, or liquid) into the instrument interior, which operates at vacuum. In an atmospheric sampling probe, the pressures encountered can be quite high (up to almost 100 bar for a Venus entry probe or lander) resulting in the need for a pressure restricting device between the ambient atmosphere and the instrument interior.
The relatively modest pumping speeds (1-5 liters/sec are typical) of conventional spaceflight vacuum pumps constrain the conductance of the pressure restrictor to values that are quite low, and in fact outside of the range that can be practically implemented using commonly-available tubing or a simple orifice. For example, using 0.0015″ (38.1 microns μm) inner diameter tubing (available as 36 gauge stainless steel hypodermic tubing) would require a coil approximately eight hundred feet long to achieve a conductance sufficiently low to drop a 100 bar ambient to the 10−5 torr range, assuming a pumping speed of 5 liter/sec.
In addition to being sufficiently low in conductance, an inlet leak for spaceflight mass spectrometry must also be chemically inert; must not distort the gas composition being sampled by adsorbing or reacting with sampled gases differentially; and must have a reasonably fast response time (on the order of seconds or less). Finally, an inlet leak for spaceflight mass spectrometry must be robust and operable over a wide temperature and pressure range.
A inlet leak for spaceflight mass spectrometry made of a high aspect ratio tube of roughly micron-scale diameter and millimeter-scale length results in conductance of a correct order of magnitude to drop pressure from 10's of bar to the 10−5 to 10−7 mbar range typical of a mass spectrometer ion source. Conventional methods of producing such leaks have included pulled glass and crimped metal tubes both of which are prone to single point failure and are produced by either a relatively costly and labor intensive manufacturing processes (in the first case) or a relatively unrepeatable one (in the second case). Significantly, clogging of a crimped metal leak by a sulfuric acid droplet on the Pioneer Venus Probe Mass Spectrometer caused that instrument's failure to collect meaningful data over a portion of its descent trajectory. Other attempts at producing a suitable pressure restrictor have used porous frits, which tend to react differentially with sampled gases, leading to poor temporal resolution since the large surface area must equilibrate with the changing ambient pressure and composition. Alternatively, the Soviets used a piezo valve that was transiently opened. The problem with this approach was that the approach it introduced a pressure burst that required time scales on the order of minutes to stabilize, thus reducing sample resolution.
For at least the reasons stated above, and for other reasons stated below, which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for systems, methods and apparatus of an inlet leak in a spaceflight mass spectrometer that are sufficiently low in conductance and chemically inert; do not distort the gas composition being sampled by adsorbing or reacting with sampled gases differentially, have a reasonably fast response time (on the order of seconds or less), are robust and operable over a wide temperature and pressure range, and occupy a small volume.